Clutches designed for use in heavy and medium duty applications commonly employ sleeves which ride on a transmission input shaft. The sleeve connects a release bearing with a clutch spring and/or clutch levers which provide the clutch engagement load against the pressure plate. The sleeve is disposed over the transmission input shaft and rotates with the spring, as well as the cover, flywheel and pressure plate of the clutch. A bushing in axial alignment with the bearing is press fit in the sleeve for unitary rotation therewith and rides directly on the transmission input shaft. Relative rotation between the sleeve and the transmission input shaft occurs when the clutch is partially or fully released. An inner race of the release bearing is connected to the sleeve for movement therewith. An outer race is prevented from rotating, and is in engagement with a clutch fork. Wear pads on the outer race are engaged by the ends of the fork's arms. The clutch fork is pivotably mounted in a clutch housing connecting an engine block and a transmission case. The clutch fork is connected with an operator controlled pedal for selective axial displacement of the bearing and sleeve and the consequent disengagement of the clutch.
The pivotal motion of the clutch fork, in combination with the frictional force between the fork arms and the wear pads, induces an "uplift" force on the bearing which is transferred through the bearing to the sleeve and to the bushing therein. The uplift force is approximately equal to the coefficient of friction between the fork arms and the pads multiplied by the normal force applied by the arms against the pads. The uplift force presses the sleeve bushing against the input shaft during clutch disengagements, thereby accelerating wear of the sleeve bushing.
Original equipment manufacturers of vehicles (OEMs) are now requiring that clutches be substantially maintenance free for extended mileage periods. To increase the life of sleeve bushings, it is highly desired to reduce the friction between the fork and the release bearing and thereby reduce the uplift force.
Clutch forks are known which employ rollers to engage the release bearings of those clutches. However, those forks are used in combination with clutch bearings which ride not on the transmission input shaft, but on a stationary quill extending from the transmission case and circumscribing but not touching the input shaft. Additionally, known roller forks are not appropriately configured to enable them to be substituted into existing clutch systems having very limited space within the clutch housing.
It is desired to provide a low friction clutch fork to release bearing interface which will reduce sleeve bushing wear. It is further desired to provide such a clutch fork and release bearing combination which fits in the space available for a conventional fork and release bearing combination.